A premix burner is part of an industrial furnace having a process chamber in which a drying or heating process is performed. The burner has a reaction zone communicating with the process chamber. A mixture of fuel and oxidant, which is known as a premix, is ignited and burned in the reaction zone to provide thermal energy for heating the process chamber. The premix is formed upon intermixing of the fuel and oxidant along flow paths that convey the fuel and oxidant to the reaction zone.
The combustion conditions in the reaction zone can be controlled by controlling the firing rate at which the premix is ignited upon entering the reaction zone. The firing rate is generally controlled by modulating the velocity at which the premix enters the reaction zone. The velocity is modulated uniformly throughout all of the premix flow paths leading to the reaction zone.
Modulating the premix flow velocity has certain limitations as a way to control the firing rate of the burner. First, the practical velocity turn-down range is limited by flashback. Flashback occurs when premix flow velocity decreases sufficiently to allow flame to propagate upstream along the flow paths leading to the reaction zone. Second, ultra low NOx emissions, and to some extent very low CO emissions, depend on excellent mixing of the fuel and oxidant forming the premix. Unfortunately, mixing quality can deteriorate as the flow path velocity and pressure drop decrease when the burner is turned down in a conventional manner.
Additionally, premix burners can amplify or cancel noise, depending in part on the velocity at which the premix flows toward and into the reaction chamber. The burner can be tuned for noise accordingly, but conventional turn-down changes the premix velocity and thus changes the noise tuning of the burner. This limits the velocity turn-down range for some noise-prone applications. The minimum velocity may thus be limited by flashback, emissions levels, and noise tuning limits.
Increasing the maximum velocity in a premix burner is one way to increase the turn-down range. Increasing the maximum velocity and reducing the size of the burner increases the turn-down range by increasing the amount of turn-up. However, increasing the turn-down range with a higher maximum velocity can significantly increase pressure requirements and, therefore, power costs. Accordingly, increasing the maximum premix flow velocity can be an expensive way to increase the turn-down range.
Conventional control of the burner firing rate can also be rather slow. The transition from a low to a high firing rate may take from thirty seconds to several minutes, depending on the speed of the fuel and oxidant control devices, and also on the ability of the ratio control system to maintain the fuel to oxidant ratio. Many low NOx burners require precise ratio control that can be maintained adequately only when the firing rate is changed slowly. This might not be suitable for applications that require a rapid firing rate response for optimum performance.